1. Field of the Invention
The present invention relates to a process for producing hydrophobic oil droplet-containing capsules having a urea-formaldehyde resin wall layer. More particularly it relates to a process for producing microcapsules which comprises polymerizing urea and formaldehyde around a previously dispersed hydrophobic oleophilic liquid to form a urea-formaldehyde wall reducing the viscosity of the capsule solution.
2. Brief Description of the Prior Art
In forming a urea-formaldehyde resin wall around a hydrophobic oleophilic liquid previously dispersed by polymerizing urea and formaldehyde, studies directed to the use of a specific high molecular weight electrolyte have already been made. For example, U.S. Pat. No. 4,001,140 reports the use of ethylene-maleic anhydride copolymers, vinyl methyl ether-maleic anhydride copolymers or polyarcylic acids. However, a microcapsule solution using these high molecular weight electrolytes generally has a high viscosity. Particularly when the pH of the microcapsule solution is neutralized, it becomes highly viscous and serious drawbacks are encountered preparing a pressure sensitive recording paper using these high molecular weight electrolytes. Upon coating a microcapsule solution onto a support such as a paper sheet or the like, as the solution stands its viscosity becomes too high so that the solution must be diluted with water. For this reason, the drying load becomes large and presents a serious problem to manufacture.
In addition, as is mentioned in U.S. Pat. No. 4,001,140, in the system in which urea is polymerized with formaldehyde in the presence of ethylene-maleic anhydride copolymers or vinyl methyl ether-maleic anhydride copolymers to form a urea-formaldehyde wall, it is difficult to clean up the matter deposited on the wall of a reaction vessel, particularly when it is dried. Such deposited matter can be washed out with warm water only with difficulty.